To improve thermal efficiency of gasoline internal combustion engines, lean burn is known to give enhanced thermal efficiency by reducing pumping losses and increasing ratio of specific heats. Generally speaking, lean burn is known to give low fuel consumption and low nitrous oxide, or NOx emissions. There is however a limit at which an engine can be operated with a lean air/fuel mixture because of misfire and combustion instability as a result of a slow burn. Known methods to extend the lean limit include improving ignitability of the mixture by enhancing the fuel preparation, for example using atomised fuel or vaporized fuel, and increasing the flame speed by introducing charge motion and turbulence in the air/fuel mixture. Finally, combustion by auto-ignition, or homogeneous charge compression ignition, has been proposed for operating an engine with very lean or diluted air/fuel mixtures. A lean air/fuel mixture has a lambda value of λ>1. By definition, a lambda value λ less than 1 is termed “rich”, while a value greater than 1 is termed “lean”. The lambda value is defined as the quantity of intake air divided by the theoretical air requirement, where the ideal stoichiometric air/fuel ratio (14.5 parts air and 1 part fuel) has a lambda value of λ=1.
When certain conditions are met within a homogeneous charge of lean air/fuel mixture during low load operation, homogeneous charge compression ignition can occur wherein bulk combustion takes place initiated simultaneously from many ignition sites within the charge, resulting in very stable power output, very clean combustion and high fuel conversion efficiency. NOx emission produced in controlled homogeneous charge compression ignition combustion is extremely low in comparison with spark ignition (SI) combustion based on propagating flame front and homogeneous charge compression ignition (HCCI) combustion based on an attached diffusion flame. In the latter two cases represented by spark ignition engine and diesel engine, respectively, the burnt gas temperature is highly homogeneous within the charge with very high local temperature values creating high NOx emission. By contrast, in controlled homogeneous charge compression ignition combustion where the combustion is uniformly distributed throughout the charge from many ignition sites, the burnt gas temperature is substantially homogeneous with much lower local temperature values resulting in very low NOx emission.
Engines operating under controlled homogeneous charge compression ignition (HCCI) combustion have already been successfully demonstrated in two-stroke gasoline engines using a conventional compression ratio. The high proportion of burnt gases remaining from the previous cycle, i.e., the residual content, within the two-stroke engine combustion chamber is responsible for providing the hot charge temperature and active fuel radicals necessary to promote homogeneous charge compression ignition in a very lean air/fuel mixture. In four-stroke engines, because the residual content is low, homogeneous charge compression ignition is more difficult to achieve, but can be induced by heating the intake air to a high temperature or by significantly increasing the compression ratio. This effect can also be achieved by retaining a part of the hot exhaust gas, or residuals, by controlling the timing of the intake and exhaust valves.
In all the above cases, the range of engine speeds and loads in which controlled homogeneous charge compression ignition combustion can be achieved is relatively narrow. The fuel used also has a significant effect on the operating range; for example, diesel and methanol fuels have wider auto-ignition ranges than gasoline fuel. A further problem is to achieve ignition at a particular time with maintained combustion stability, while avoiding engine knocking and misfiring. This is a particular problem when operating the engine under HCCI combustion at relatively high load in a region where a mode switch from HCCI to SI may be required.
In order to extend the operating range of an engine operated in HCCI-mode the intake pressure can be boosted by means of a turbocharger or a compressor. This allows the operating range, or operational window, to be extended. However, when switching between a combustion mode using a higher manifold pressure (charged or boosted) to a combustion mode using a lower manifold pressure (ambient or throttled), a problem arises in evacuating the excess air in the manifold in a very short time. In normal SI-mode the engine operates at or near a lambda value of λ=1 and a leaner combustion, as used in HCCI-mode, could give problems in combustion stability and or exhaust after treatment e.g. NOx emissions. Secondly, a mode change should occur as fast as possible in order to avoid intermediate combustion modes that are difficult to control.
The problem of evacuating the excess air in the manifold when switching between operating modes also occurs when switching from stratified to homogenous combustion when operating an engine in SI-mode.
Hence, the inventors recognized a need for solving the problem of controlling a supercharged engine and evacuating the excess air in the manifold when switching from a combustion mode using a higher manifold pressure, such as charged or boosted, to a combustion mode using a lower manifold pressure, such as ambient or throttled.
Accordingly, the present invention is directed to an automotive system, including: an internal combustion engine, comprising: at least one cylinder; a piston whose compression action causes a mixture of air and fuel within a combustion chamber of said cylinder to be ignited; at least one inlet valve for admitting gas which includes fresh air into said cylinder; at least one exhaust valve for exhausting combusted gases from said cylinder a fuel injection system having at least one fuel injector, through which fuel is injected into said combustion chamber of said cylinder; an intake air charging system for supplying air under pressure to an engine air intake manifold; at least one sensor for measuring an engine operating parameter; and a controller operating said engine in a first combustion mode, said controller subsequently switching engine operation to a second combustion mode wherein manifold pressure in said second combustion mode is lower than manifold pressure in said first combustion mode; and controlling said intake air charging system to cause a temporary airflow reversal thereby equalizing a pressure difference between said intake valve and said exhaust valve.
The present invention relates to an internal combustion engine provided with a system for boosting the manifold absolute pressure (turbocharger, compressor etc.). In addition the engine is preferably, but not necessarily, provided with means for variable valve timing (VVT) and cam profile switching (CPS). Examples of full variable valve systems are e.g. electrical magnetic valve systems and electrical hydraulic valve systems.
Also, although the following examples relate to gasoline fuels, an engine operating according to principles of the invention can be adapted to use most commonly available fuels, such as diesel, kerosene, natural gas, and others.
A reciprocating piston is arranged in each engine cylinder whose compression action causes a mixture of air and gasoline fuel within the combustion chamber to be ignited. Gas exchange is controlled by at least one inlet valve preferably, but not necessarily, provided with variable valve timing per cylinder for admitting a combustible gas, such as air, and at least one exhaust valve preferably, but not necessarily, provided with variable valve timing per cylinder for exhausting combusted gases.
The combustion process is monitored by sensors for measuring engine knocking and combustion stability. The knock sensor can be of the piezoelectric type, which may also be used for continuous monitoring of cylinder pressure. The combustion stability sensor may be an acceleration type sensor, such as a flywheel sensor, or an ion current sensor. Alternatively, both said sensors can be replaced by a single in-cylinder piezoelectric pressure sensor. By processing the output from such a sensor, it is possible to obtain a signal representing engine knock and a signal representing engine stability.
According to one example, the engine is possible to be operated in homogeneous charge compression ignition (HCCI) combustion mode. In the subsequent text, this will be referred to as HCCI-mode or compression ignition mode. This is a combustion mode, different from a conventional spark ignition (SI) combustion mode, used in order to reduce fuel consumption in combination with ultra low NOx emissions. In this mode, a mixture containing fuel, air and combustion residuals is compressed with a compression ratio between 10.5 and 13 to auto ignition. The HCCI combustion has no or a very slow moving flame front, in contradiction to a SI combustion that has a moving flame front. The lack of a flame front reduces temperature and increases the heat release rate, which in turn increases the thermal efficiency of the combustion. The stoichiometric mixture must be diluted with access air and or residuals in order to reduce the heat release rate. This reduces both pumping losses and combustion temperature hence the fuel consumption compared to an SI operated engine. The combustion residuals are captured when operating the engine with a negative valve overlap. Residuals increase the temperature of the mixture so that the auto ignition temperature is reached before piston top dead centre (TDC) and dilute the mixture so that the heat release rate decreases to an acceptable level. By controlling the heat release, cycle-to-cycle variations (COV), noise and knocking combustion can be reduced. The negative valve overlap is achieved when the exhaust valve is closed before piston TDC and the inlet valve is opened after piston TDC in the gas exchange phase of the combustion.
The acquired valve timing for the negative overlap can be achieved by using suitable fully or partially variable valve systems (VVT), and CPS, hence switching from conventional SI valve timing to HCCI valve timing with a shorter the valve opening duration and/or valve lift.
During compression ignition mode, the exhaust valve is arranged to be closed before top dead centre during an exhaust stroke of the piston and the intake valve is arranged to be opened after top dead centre during an induction stroke of the piston, in order achieve a negative valve overlap to retain residual exhaust gas. The control unit may be arranged to control the fuel injection system so as to perform one or more fuel injections depending on the current operating conditions. The general operation of an engine in the HCCI-mode is well known in the art and will not be described in further detail.
A further preferred embodiment of the invention relates to an internal combustion engine, comprising the component parts as described above, provided with at least one cylinder and arranged to be switched between stratified and homogenous combustion in spark ignition mode.
During stratified combustion fuel is injected directly into the cylinder and the stratified, rich mixture is contained near the spark plug where it is ignited. This combustion mode is used for providing an engine that may achieve both the fuel efficiency of a diesel engine and the high output of a conventional petrol engine. The advantage of this stratified operating mode is the reduction in charge cycle losses caused by operation without throttle, or wide open throttle, in part-load conditions. In this case, fuel is the only factor controlling performance. With the homogeneous (λ=1) operating mode, the fuel is injected during the induction stroke and is mixed with induction air throughout the combustion chamber. This mode can be used across the entire speed/load range. In this case, as with intake manifold injection, performance is controlled by charging and ignition. With the homogeneous lean (λ>1) operating mode (as with the “homogenous” mode); injection takes place during the induction stroke. The quantity of fuel is, however, less (excess air). As with stratified mode, fuel is also the factor controlling performance in this operating mode.
For both the above embodiments, the invention provides an improved method for evacuating the excess air in the manifold when switching between operating modes. This is achieved by controlling a supercharged engine and evacuating the excess air in the intake manifold when switching from a combustion mode using a higher manifold pressure (charged or boosted) to a combustion mode using a lower manifold pressure (ambient or throttled).
When sensing a load change for the engine load or engine speed that requires a combustion mode switch, the controller switches the engine from said first combustion mode using a higher intake manifold pressure to said second combustion mode using a lower intake manifold pressure, and further controls the intake air charger to cause a surge in the intake air in order to evacuate, or equalize the higher manifold pressure.
The controller is adapted to switch the intake air charger from a part load map to a map corresponding to a higher load map, such as a full load map. By disabling the load map, for instance by disabling the vane function of a variable diffuser vane turbocharger compressor, a surge is initiated. The concept of load maps for air chargers such as compressors will be described in further detail below. As soon as the excess air has been evacuated from the intake air manifold and the pressure has been reduced to ambient or throttled pressure, the surge is interrupted by switching the compressor back to a load map that will not induce surge and opening a wastegate to prevent the pressure from increasing in the intake air manifold. As stated above, the invention is preferably directed to a direct fuel injection (DI) engine, but it may also be used for engines with port injection. In the latter case, it should be ensured that the excess air to be evacuated from the intake air manifold does not contain injected fuel.
In principle, the intake air charger may be any type that is susceptible to surge, whereby air flow is temporarily reversed. The intake air charger used for the invention is preferably, but not necessarily, a variable geometry compressor or a variable turbocharger compressor. Examples of such air charging means may be compressors with variable diffuser vanes, a variable nozzle or with swirl reversal vanes.
The above advantages and other advantages, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings, and from the claims.